Biosphere, relatively thin life-supporting stratum of Earth’s surface, extending from a few kilometres into the atmosphere to the deep-sea vents of the ocean. The biosphere is a global ecosystem composed of living organisms (biota) and the abiotic (nonliving) factors from which they derive energy and nutrients.

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Before the coming of life, Earth was a bleak place, a rocky globe with shallow seas and a thin band of gases—largely carbon dioxide, carbon monoxide, molecular nitrogen, hydrogen sulfide, and water vapour. It was a hostile and barren planet. This strictly inorganic state of the Earth is called the geosphere; it consists of the lithosphere (the rock and soil), the hydrosphere (the water), and the atmosphere (the air). Energy from the Sun relentlessly bombarded the surface of the primitive Earth, and in time—millions of years—chemical and physical actions produced the first evidence of life: formless, jellylike blobs that could collect energy from the environment and produce more of their own kind. This generation of life in the thin outer layer of the geosphere established what is called the biosphere, the “zone of life,” an energy-diverting skin that uses the matter of the Earth to make living substance.

The biosphere is a system characterized by the continuous cycling of matter and an accompanying flow of solar energy in which certain large molecules and cells are self-reproducing. Water is a major predisposing factor, for all life depends on it. The elements carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, when combined as proteins, lipids, carbohydrates, and nucleic acids, provide the building blocks, the fuel, and the direction for the creation of life. Energy flow is required to maintain the structure of organisms by the formation and splitting of phosphate bonds. Organisms are cellular in nature and always contain some sort of enclosing membrane structure, and all have nucleic acids that store and transmit genetic information.

All life on Earth depends ultimately upon green plants, as well as upon water. Plants utilize sunlight in a process called photosynthesis to produce the food upon which animals feed and to provide, as a by-product, oxygen, which most animals require for respiration. At first, the oceans and the lands were teeming with large numbers of a few kinds of simple single-celled organisms, but slowly plants and animals of increasing complexity evolved. Interrelationships developed so that certain plants grew in association with certain other plants, and animals associated with the plants and with one another to form communities of organisms, including those of forests, grasslands, deserts, dunes, bogs, rivers, and lakes. Living communities and their nonliving environment are inseparably interrelated and constantly interact upon each other. For convenience, any segment of the landscape that includes the biotic and abiotic components is called an ecosystem. A lake is an ecosystem when it is considered in totality as not just water but also nutrients, climate, and all of the life contained within it. A given forest, meadow, or river is likewise an ecosystem. One ecosystem grades into another along zones termed ecotones, where a mixture of plant and animal species from the two ecosystems occurs. A forest considered as an ecosystem is not simply a stand of trees but is a complex of soil, air, and water, of climate and minerals, of bacteria, viruses, fungi, grasses, herbs, and trees, of insects, reptiles, amphibians, birds, and mammals.

Stated another way, the abiotic, or nonliving, portion of each ecosystem in the biosphere includes the flow of energy, nutrients, water, and gases and the concentrations of organic and inorganic substances in the environment. The biotic, or living, portion includes three general categories of organisms based on their methods of acquiring energy: the primary producers, largely green plants; the consumers, which include all the animals; and the decomposers, which include the microorganisms that break down the remains of plants and animals into simpler components for recycling in the biosphere. Aquatic ecosystems are those involving marine environments and freshwater environments on the land. Terrestrial ecosystems are those based on major vegetational types, such as forest, grassland, desert, and tundra. Particular kinds of animals are associated with each such plant province.

Ecosystems may be further subdivided into smaller biotic units called communities. Examples of communities include the organisms in a stand of pine trees, on a coral reef, and in a cave, a valley, a lake, or a stream. The major consideration in the community is the living component, the organisms; the abiotic factors of the environment are excluded.

A community is a collection of species populations. In a stand of pines, there may be many species of insects, of birds, of mammals, each a separate breeding unit but each dependent on the others for its continued existence. A species, furthermore, is composed of individuals, single functioning units identifiable as organisms. Beyond this level, the units of the biosphere are those of the organism: organ systems composed of organs, organs of tissues, tissues of cells, cells of molecules, and molecules of atomic elements and energy. The progression, therefore, proceeding upward from atoms and energy, is toward fewer units, larger and more complex in pattern, at each successive level.

The biosphere supports between 3 and 30 million species of plants, animals, fungi, single-celled prokaryotes such as bacteria, and single-celled eukaryotes such as protozoans (Figure 1). Of this total, only about 1.4 million species have been named so far, and fewer than 1 percent have been studied for their ecological relationships and their role in ecosystems. A little more than half the named species are insects, which dominate terrestrial and freshwater communities worldwide; the laboratories of systematists are filled with insect species yet to be named and described. Hence, the relationships of organisms to their environments and the roles that species play in the biosphere are only beginning to be understood.

The organization of the biosphere

Natural groupings

This tremendous diversity of life is organized into natural ecological groupings. As life has evolved, populations of organisms have become separated into different species that are reproductively isolated from one another. These species are organized through their interrelationships into complex biological communities. The interactions in these communities affect, and are affected by, the physical environments in which they occur, thereby forming ecosystems through which the energy and nutrients necessary for life flow and cycle. The mix of species and physical environments vary across the globe, creating ecological communities, or biomes, such as the boreal forests of North America and Eurasia and the rainforests of the tropics. The sum total of the richness of these biomes is the biosphere.

Processes of evolution

This hierarchical organization of life has come about through the major processes of evolution—natural selection (the differential success of the reproduction of hereditary variations resulting from the interaction of organisms with their environment), gene flow (the movement of genes among different populations of a species), and random genetic drift (the genetic change that occurs in small populations owing to chance). (Seeevolution.) Natural selection operates on the expressed characteristics of genetic variants found within populations, winnowing members of the population who are less well suited to their environment from those better suited to it. In this manner, populations become adapted to their local ecosystems, which include both the physical environment and the other species with which they interact in order to survive and reproduce.

The genetic variation that is necessary for a species to adapt to the physical environment and to other organisms arises from new mutations within populations, the recombination of genes during sexual reproduction, and the migration of and interbreeding with individuals from other populations. In very small populations, however, some of that variation is lost by chance alone through random genetic drift. The combined result of these evolutionary processes is that after many generations populations of the same species have widely divergent characteristics. Some of these populations eventually become so genetically different that their members cannot successfully interbreed, resulting in the evolution of a separate species (speciation).

The diversification of life through local adaptation of populations and speciation has created the tremendous biodiversity found on the Earth. In most regions a square kilometre (0.4 square mile) will harbour hundreds—in some places even thousands—of species. The interactions between these species create intricate webs of relationships as the organisms reciprocally evolve, adapting to one another and becoming specialized for their interactions (coevolution; seecommunity ecology: The coevolutionary process). Natural communities of species reflect the sum of these species’ interactions and the ongoing complex selection pressures they constantly endure that drive their evolution. The many ecological and evolutionary processes that affect the relationships among species and their environments render ecology one of the most intricate of the sciences. The answers to the major questions in ecology require an understanding of the relative effects of many variables acting simultaneously.